A few months back I was at the main Apple Store in New York City. I wanted to buy a case for my son’s iPod touch—but it was December 23. The crowds were so thick, I envied sardines. Fortunately, I knew something that most of these people didn’t: I could grab an item off the shelf, scan it with my iPhone and walk right out. Thanks to the free Apple Store app, I didn’t have to wait in line or even find an employee. The purchase was instantly billed to my Apple account. I was in and out of there...